In Spanish, the verb endings of the different conjugations show which grammatical person they agree with (who is doing the action). So, it is not always necessary to include them in sentences because you might sound redundant and repetitive.
Explanation:
We mostly use the subject pronouns for emphasis or change of subject, for example:
In Spanish, the verb forms often make subject pronouns unnecessary, and when that's the case the pronouns shouldn't be used unless there's a reason to.
